The present invention relates to thermal transfer recording method and apparatus in which an image transferred to an intermediate transfer recording medium by utilizing a thermal transfer recording system is further transferred to a transfer-receiving material, thereby transferring and recording the image to the transfer-receiving material, and more particularly, relates of thermal transfer recording method and apparatus capable of well reproducing a color image having an improved color discrepancy.
In the known art, there have been provided various thermal transfer recording methods in which a thermal transfer sheet provided with a substrate sheet having one surface on which a color material layer is formed and a transfer-receiving material having a receptor layer, as occasion demands, are press-contacted between a heating device such as thermal head and a platen roller, a heating portion of the heading device is selectively heated in response to an image information, and a color material of the color material layer is transferred to the transfer-receiving material, thereby recording the image having predetermined information. In these thermal transfer recording methods, in recent years, a heat sensitive fusion transfer system and a heat sensitive sublimation transfer system have been most widely utilized.
The heat sensitive fusion transfer system is an image recording method which utilizes a thermal transfer sheet provided with a substrate sheet such as plastic film having a heat fusible ink layer, as a color material layer, formed by dispersing the color material such as pigment into a heat fusible binder such as wax or resin, and in which a thermal energy in response to an image information is applied by a heating device such as thermal head, to thereby transfer the color material together with the binder onto a transfer-receiving material such as paper or plastic sheet. An image obtained through this heat sensitive fusion transfer method has a high density and an excellent color clearness and thus, is suitable for the recording of binary images such as letters or lines.
On the other hand, the heat sensitive sublimation transfer system is an image recording method which utilizes a thermal transfer sheet provided with a substrate sheet such as plastic film having a dye layer, as a color material layer, formed by fusing or dispersing a sublimate dye as color material into a binder resin and a transfer-receiving material provided with a support member such as paper or plastic sheet having a receptor layer for color material, and in which a thermal energy in response to an image information is applied by a heating device such as thermal head, to thereby transfer only the color material in the color material layer of the thermal transfer sheet onto the receptor layer of the transfer-receiving material to thereby record the image.
In these transfer methods, a multi-color or color image can be recorded by utilizing thermal transfer sheets such as yellow, magenta, cyan and black color sheets and recording on the transfer-receiving material in an overlapped manner.
In these transfer recording systems, however, particularly, in the heat sensitive sublimation transfer system, it is necessary for an image formation surface of the transfer-receiving material to have a dying property to a dye as a color material, and it is almost impossible to form an image on the transfer-receiving material provided with no receptor layer having the dying property. In such view, in order to form, through the heat sensitive sublimation transfer system, an image to the transfer-receiving material other than a specific paper preliminarily formed with the receptor layer, the prior art provides a technique in which a receptor layer transfer sheet having a substrate film to which a receptor layer is formed to be peelable therefrom is prepared, and the receptor layer is transferred to the transferreceiving material, and the color material is transferred from the thermal transfer sheet onto the transferred receptor layer, thus forming the image on the transfer-receiving material. Such technology is disclosed, for example, in Japanese Patent Laid-open Publication No. SHO 62-264994. According to this transfer system, the receptor layer which has been transferred to the transfer-receiving material is largely subjected to an influence with a surface quality of the transfer-receiving material. That is, there may be caused a problem in which the receptor layer is not formed to a recessed portion of the surface of the transfer-receiving material or the receptor layer becomes irregular because of an irregularity of the surface of the transfer-receiving material, leading to irregularity of a formed image. Accordingly, in the known art, it is necessary to select the transfer-receiving material having a flat smooth surface condition to obtain a desired fine image.
Thus, in order to prevent the adverse influence to the image quality with the surface irregularity or surface condition of the transferreceiving material and to make possible the formation of the image on an optional transfer-receiving material, in the known art, an intermediate transfer recording medium, in which the receptor layer is formed on a substrate sheet to be peelable, is first prepared, an image is formed on this receptor layer through the heat sensitive sublimation transfer system with the use of the thermal transfer sheet, and the intermediate transfer recording medium formed with such an image is then overlapped with the transfer-receiving material and heated to thereby transfer the receptor layer in which the image is already formed onto the transfer-receiving material. Such technology is disclosed, for example, in Japanese Patent Laid-open Publication No. SHO 62-238791.
FIG. 20 is an illustration showing a schematic structure of a conventional thermal transfer recording apparatus 200 utilizing the intermediate transfer recording medium mentioned above. With reference to FIG. 20, an intermediate transfer recording medium 1 having a long scale and a thermal transfer sheet 2 also having a long scale are fed and conveyed from supply rolls 31 and 33, respectively, press-contacted together by a first thermal head 4 and a first platen roller 5 at a printing section, and a thermal energy is applied in accordance with an image information by the first thermal head 4. Through these processes, a color material of the thermal transfer sheet 2 is transferred to a receptor layer of the intermediate transfer recording medium 1 to form an image A, and the thermal transfer sheet 2 is thereafter rolled up around a wind-up roll 34. Then, the intermediate transfer recording medium on which the image A has been formed is continuously conveyed to a transfer section, in which the intermediate transfer recording medium and the transfer-receiving material B are pressed together by a second thermal head 4a and a second platen roller 5a. Through the heating process of the second thermal head 4a, the receptor layer on which the image has been formed is transferred to the transfer-receiving material B, and after this transfer process, the intermediate transfer recording medium is wound up around the wind-up roll 32. Further, it is to be noted that although transferring width and length of the second thermal head 4a as a heating means for transferring the receptor layer from the intermediate transfer recording medium to the transfer-receiving material can be optionally set, a heating roller may be utilized in a case where optional setting to every image is not required.
Incidentally, in order to accommodate the intermediate transfer recording medium in a thermal transfer recording apparatus as much in volume as possible and to reduce a material cost, it is desired to use a thin film as a substrate sheet of the intermediate transfer recording medium. However, when the thin film is used, because the thin film is generally manufactured by being elongated in vertical and horizontal directions, a thermal shrinkage will be caused when exposed to high temperature condition after the manufacture thereof. A like phenomenon will be caused in a case where the thin film is used for the substrate sheet of the intermediate transfer recording medium, and a thermal shrinkage will be also caused through the heating by the thermal head at a time when an image is formed to the receptor layer of the intermediate transfer recording medium. Particularly, in the recording of a color image, since the color image is displayed by overlapping respective images of yellow, magenta and cyan colors, it is required to accurately accord with the respective color images in positions, and if such positioning is not accurate, a displayed image provides a bad appearance. If the substrate sheet of the intermediate transfer recording medium is shrunk every time when the images of the respective colors are formed on the receptor layer of the intermediate transfer recording medium, the respective color images differ in their sizes and it becomes impossible to accurately overlap these images with each other.
Although the shrinkage or elongation in the length direction, i.e. conveying direction of the intermediate transfer recording medium, can be controlled by adjusting tension to be applied in the length direction thereof at the conveying time, any tension is not applied, in usual, in the width direction, i.e. a direction normal to the conveying direction, it is difficult to prevent the shrinkage or elongation of the substrate sheet in the width direction. For example, in a case where a three-color (YMC) image is formed to the intermediate transfer recording medium using a polyethylene terephthalate film having a thickness of 10 .mu.m, shrinkage of about 0.6-1.0 mm is caused with respect to the width of 180 mm, which corresponds to the shifting of 8-13 dots at a picture element density of 12 dots/mm, and this shifting amount can be not neglected for the color reproduction of a clear image.